Transistor amplifier circuits



Sept. 15, 1953 R. L.. WALLACE, JR

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A TTORA/EV `Patented Sept. 15, 1953 NITED STATES @idi'iiNT OFFICE TRANSISTOR AMPLIFIER CIRCUITS Application September 12, 1950, Serial No. 184,457

Claims.

This invention relates to transistor translating circuits.

lThe general objects of the invention are to provide novel transistor circuits and particularly transistor amplifier circuits of improved performance. Those transistor circuits so provided which are chosen for illustration exemplify the central principle of the invention and the design principles according to which not only these illustrative circuits, but many others, too, are derived and may be constructed.

Another particular object is to provide an improved multistage transistor amplifier.

A related object is to provide an improved interstage coupling network for a multistage transistor amplifier.

Another related object is to provide automatic adjustment of the emitter bias current of a transistor amplifier.

Another particular object of the invention is to provide an efficient class B push-pull transister translating circuit.

The transistor, which is the subject of a patent application of John Bardeen and W. H. Brattain, Serial No. 33,466, filed June 17, 1943, now Patent 2,524,035, issued October 3, 1950, is a threeelectrode device capable of amplifying electric signals. Upon the announcement of the invention of the transistor, it was generally treated as analogous to a vacuum tube and efforts were made to amplify and otherwise translate electric signals by means of conventional circuits whose performance in connection with vacuum tubes had become well known, the only change made being to substitute a transistor for the vacuum tube. These efforts were often of doubtful success, and the reason was believed to be that the transistor was at best a very imperfect analog of the vacuum tube.

Among the various points of departure of the transistor from perfect analogy with the vacuurn tube, those principally remarked upon have been its low input impedance and the fact that it is essentially a current-operated device, as compared with the high input impedance and voltage operation of the Vacuum tube triode. It was believed that such points of dissimilarity constituted defects, perhaps temporary only, in the transistor, and it was hoped that improvements in fabrication procedures Would result in transisters which should be more nearly perfect analogs of the vacuum tube, in which case improved performance in conventional circuits might be expected. At the same time a number of novel circuits were constructed which appeared to take advantage of the peculiar characteristics of the transistor, but the search for such new circuits was necessarily conducted in a somewhat haphazard fashion.

The present invention is based upon the realization that the transistor approximates the dual counterpart of a vacuum tube triode much more closely than it approximates the analog of the tube, and that, in fact, it approximates the tube dual very closely indeed; and that this duality relation holds not only in a qualitative sense but in a quantitative sense as well, as may be immediately seen by comparison of the transister collector voltage-current characteristics for various values of emitter current with the tube anode current-voltage characteristics for various values of the grid voltage. Thus, generally speaking, the more imperfect the parallel on the analogy basis, the more reliable and complete is the parallel on the duality basis.

The invention is further based upon the realization that when excellent performance is known to be obtainable from a particular circuit coniiguration of which a vacuum tube is a part, then comparable performance can be expected from a transistor circuit which is the dual of the known vacuum tube circuit, and of which the transistor, itself an approximate dual of the vacuum tube, forms a part.

As an example of the application of the foregoing principles and, incidentally, one in connection with which they were formulated, consider a class B push-pull transistor amplier circuit. In the well-known class B push-pull vacuum tube amplier circuit, each of the two tubes is biased substantially to its anode current cut-ofi, so that, in the absence of a signal, it dissipates only a negligible amount of power despite the fact that the quiescent anode voltage is high; and, when the signal is impressed on the input circuit, the two anode currents iiow in alternation, wide swings of current and voltage being possible Without overloading. When it was attempted to construct a push-pull transistor amplifier and to bias it for class B operation in this it was found to be very inferior; so much so that, instead of delivering several times the output power obtainable with an unbalanced amplifier circuit employing a single transistor, as was to be expected by analogy, it invariably delivered much less.

When, however, the transistor circuit was redesigned as the dual of the conventional vacuum tube circuit, the emitter current being treated as the bias control and the emitter current and plate current of each transistor being adjusted toward collector voltage cut-off, it was found to give excellent performance; indeed, it delivered twenty to forty times as much power as did the analog class B transistor amplier. This serves to confirm the soundness of the general principles outlined above and formulated in detail below. These principles facilitate the design of large numbers of transistor circuits which are dual to known vacuum tube circuits and, to some extent., prediction of their performance.

In another aspect the invention provides an improved multistage transistor amplifier and frequency-selective interstage coupling networks for it which are the dual counterparts of the coupling networks commonly employed with conventional vacuum tube amplifiers, having low impedances within the pass band and higher irnpedances outside of it, where the common ones have high impedances in the band and higher impedances outside of it. In the simplest case the network is a series-resonant circuit as compared with the common parallel-resonant or antiresonant circuit of the vacuum tube amplifier.

In still another aspect the invention provides a combination of an inductive element and a resistor connected to the emitter of a transistor in such a fashion as to automatically adjust the emitter bias current to a Value suitable for the input signal. This action is the dual counterpart of that of the grid-leak-condenser combination which is commonly employed for automatic control of the grid voltage bias of a vacuum tube.

The invention will be fully apprehended from the following detailed description of the principles upon which it is based and of certain specic embodiments thereof taken in connection with the appended drawings, in which:

Figs. 1a and 1b shown a family of conventional vacuum tube voltage-current characteristics and a family of transistor current-voltage characteristics, placed side by side for comparison;

Figs. 2a and 2b are circuit diagrams showing two passive networks each of which is the dual of the other, together with their defining equations;

Fig. 3a is a schematic circuit diagram showing a conventional vacuum tube amplifier while Fig. 3b shows its dual counterpart, a transistor amplier, the defining equations of each being set forth side by side for comparison;

Fig. 3c is a diagram illustrating the equivalence of Thevenins theorem;

Fig. 4 is a schematic circuit diagram of a transistor amplifier embodying the principles of Fig. 3b and with a low resistance load;

Fig. 5 is a schematic circuit diagram of a transistor amplier embodying the principles of Fig. 3b and with a tuned circuit coupling to a load;

Fig. 6 shows two families of transistor characteristics each of which is the same as that of Fig. lb, and juxtaposed back to back for illustration of class B push-pull operation;

Figs. 7a and 7b are schematic circuit diagrams of a vacuum tube push-pull amplifier and of its dual counterpart, a transistor push-pull amplier, the defining equations of each being placed side by side for comparison;

Fig. 8 is a schematic circuit diagram of a practical class B push-pull transistor amplifier, working into a resistive load;

Fig. 9 is a schematic circuit diagram showing a modification of Fig. 8 in which all bias currents are supplied from a common source;

e Ri z' Gc and di de G L where all of the symbols have their conventional meanings. Any quantity which in this sense is the reciprocal of another quantity is said to be the dual of that quantity. These equations, taken in pairs, are duals, and they show that complete duality exists between voltage, current, and the electric circuit elements.

We have Quantity Dual Quantity Voltage Current. Resistance Conuuciauce. Inductauce Capacitance.

The vacuum tube is essentially a voltage-amplifying device While the transistor is essentially a current-amplifying device. This fact, which has been recognized for some time, hints that the relation between vacuum tubes and transistors is not one of similarity but rather of duality; that is, that the roles of currents and potentials in the transistor are justl interchanged by comparison with their roles in the Vacuum tube. Figs. la and 1b illustrate and confirm this statement. They show a family of static characteristics of a vacuum tube, as widely published in texts and handbooks, plotted beside a corresponding family of N- type transistor characteristics, as published, for example, by R. M. Ryder and R. J. Kircher in Some Circuit Aspects of the Transistor, Bell System Technical Journal, July 1949, page 367 (vol. 28). When the axes are chosen in the manner shown, the two families of curves are almost identical in shape. The quantities which behave similarly are ep and -z'c 1p and -ec cg and ie, and, approximately, ig and ee The consistent difference in sign is of no signicance with respect to duality considerations because it could be removed by a reversal of all the sign conventions for the transistor; and in fact the signs are all reversed for a P-type transistor. Comparison of the two families of characteristics of Figs. la and 1b indicates that the transistor collector circuit is anapproximate dual of the vacuum tube plate circuit and that the emitter circuit is an approximate dual of the vacuum tube grid circuit. In particular, they show that the base, the emitter and the collector electrodes of the transistor correspond, dualitywise, to the cathode, the grid, andthe anode of the tube.

Now that this dual relationship has been found, a problem arises as to what it means with respect to the design and performance of transistor translating circuits. Since the transistor is not so much the analog of the vacuum tube as its dual, it may be supposed that transistor circuits should not be similar to vacuum tube circuits but rather dual to them. This implies that if it is desired to duplicate, with a transistor, the performance of a known vacuum tube circuit, what is called for is not merely to remove the vacuum tubes and replace them with transistors but rather to first alter the circuit in such a way that the roles of currents and potentials are interchanged in all the passive elements of the circuit as well as in the active ones.

After such circuits have been found, the operating biases should be chosen in such a way as to take into account the following dual situations, stated with respect to N-type transistors: (With P-type transistors, the signs of all biases are to bc reversed.)

I. Biasing the vacuum tube grid positively with respect to its cathode so that grid current begins to flow corresponds to biasing the transistor emitter negatively with respect to its base so that negative emitter potential begins to increase.

Il. Biasing the vacuum tube grid sufliciently negative to reduce anode current essentially to zero corresponds to biasing the transistor emitter sufficiently positive to reduce collector voltage essentially to zero.

III. Biasing the vacuum tube anode negatively so that anode current is reduced to zero corresponds to biasing the transistor collector positively so that the collector voltage is reduced approximately to Zero.

The dual of a simple ladder network The foregoing may be illustrated by the design of the dual of the simple ladder network of Fig. 2a. The rst step in finding the dual is to Write down the Kirchhoi equations for the circuit. These are and il-iE-zs-:O

Now, in these equations every i is to be replaced by e/r and every e by Ti. The quantity r is a constant of the transformation which in this case can be given any positive or negative value. The eiect of r is to determine how many volts in the dual circuit are equivalent to one ampere in the original. If the circuit includes a vacuum tube then the value of r is xed by the relation between the vacuum tube quantities and the corresponding quantities of the transistor which is to replace the tube. In this case where rc is the collector resistance of the transistor and rp is the plate resistance of the vacuum tube.

When equations (1) are transformed as indicated, they become 6. dual circuit. A circuit which win nt them must now be found. To do so, it may be noted that the term eI/(T2R1) denotes the current through a resistance of value R1=r2/R provided e1 is interpreted as the voltage drop across this resistance. The dual circuit must then contain this resistance R1' with a voltage drop e1 across it. Carrying this reasoning through for the other terms in the iirst two of equations (3) leads to the conclusion that the dual circuit contains the following elements:

R1' :T2/R1 R,'=r2/R, 4) c'=L/r2 L'=r2C From the foregoing it may be seen that in passing from any circuit to its dual, every voltage is replaced by a current, every current by a voltage, every resistance by a conductance and vice versa, every inductance by a capacitance and vice versa. Equations (3) can now be written in a simpler notation as follows:

Rf-Moc] -0'+L'+1z2f=0 (5);

e1-e2-e3=0 where ic, means the current through a capaci-- tance of value C etc.

All the elements in the dual circuit are now known and the Kirchhoff equations, (5), tell how these elements must be connected together; i. e., they are to be interconnected as shown in Fig. 2b.

The notation employed in Equations (5) is a very useful one, and now that more is known about how the transformation will turn out, a substantial saving of effort can be effected by applying this same notation to the original equations (1).

Let eL stand for the voltage across L (measured in such a direction that iL flows from to and let a similar notation be employed for every e. Equations (l) and their duals (5) thus become It may be noted that a fourth equation has been added to express the fact that the current through C is the same as that through R2. The need for this can be avoided if the notation is extended somewhat to include terms such as 7:RES

which denotes the current which ows through 2R,2 and C and implies that the current through The dual of a single R-'C' coupled amplifier stage The procedure for finding thetransistordual of a circu-itwhich contains avacuum tube triode is Ythe same asy that described for a` passive` net. work except that the following. additional substitutionsmust be made;

Fig.- 3a .showsa-vacuum tube amplier circuit of conventional design and four of the equations which describeit. Fig. 3b show-s the transformed equations anda transistor circuit which satisies them. Here,- as in` other-rieures to follow, the

semiconductive-.bodyl ofthe transistor is represented by' a thin rectangle I, its base electrode by a heavy line'Z, its ernitterelectrode bya thin wire 3 bearingj an arrowheadl pointed toward-thebody, and lying at an angle-with-the bodysurface, and itscollector--electrode by another thin wire 4 at an equal and opposite angle butv without an arrowhead. As in the-case of'rFigszZa-and 2b, theconstant voltagesource EB which supplies operating biasyoltage-to` the vacuum tube anode circuit has been-transformedl into a constant current source IC which supplies bias current to the transistor. collector, and the :.conven.-

tional symbol for a constant voltage source such as a battery isreplaced, in Fig. Sband in other gures to follow, by aconventionalized. box-.5,v 6:.

containing a capital letter I, designating a constant current, together `with a distiguishingsubscript andan arrow to indicateV its direction of ow. source E has beentransformed into, an emitter bias constant current source Ie. These constant..

bias currents are not to be confused with vthe actual collector and emitter currents. The actual emitter current is in fact equal to the sum of the emitter bias current Ie and the .current which. ilows through the resistor R1 connected;

in parallel with the current source, While the actual collector current is in fact equal to the sum of the collectorI bias current IC and thecurrent which flows throughtheresistor- Rg, connected.v

in parallel with the currentsource. Theserelations are stated mathematically in the second and third equations under the ligure.

Now it is Well knowndnelectric .circuit .analyf A sis that the parallel combination of a sourcev of current I with a resistor Ris, from the stand' point of external measurements, equivalent to .the

series combination of a source of voltage E with Similarly the grid bias constant voltage.

the same resistor R, providedthe magnitude of theyoltage source is chosen to satisfy the relation E=IR (9) whichis known as Thevenins theorem. This equivalence is depicted in Fig. 3c.

Because constant VoltageV sources are more common and less costly than constant current sources, iti is usually preferred, as a practical matter, to realize the transistor current supplies by Way of the. circuit shown to the right in Fig. 3c, than by the one shown to the left. Furthermore this arrangement involves a `smaller consumption of direct-'current'power in the resistor R. Accordingly, th'ef parallel constant current sources of Fig. 3b. are replaced in Fig. 4 by series constant voltage sources Ee and Ec whose magnitudes ,satisfy the relation 9). Just as in the conventional vacuum tube amplifier of Fig. 3a, for high' voltage gain at the output terminals the resistor P,2 should be of large resistance and the anode voltage supply EB should be of high potential, so in Fig. 319, for high current gain the resistor RL. should be of 10W resistance, and the injected current IC should be large. In Fig. 4 these resultsare obtained by the use of a low resistance load 'r' in series with a battery li.C which satislies the relation (9) and a resistor for controlling th'e magnitude of thebias current which resistor, however, is usually of negligible value.

Similarly, it is common in the vacuum tube amplifier art to replace the resistor R2 of Fig. 3a by a parallel tuned circuit. Duality calls for a corresponding replacement of the resistor R2 in the output circuit of Fig. 4 by a low impedance reactive circuit, and this is physically realizable by way-of a series-tuned circuit comprising a coil S and. a condenser 9 as shown. in Fig. 5. Because thecondenser operates not only as a tuning element but as a blocking element as well, the transistor collectorv biasrcurrent supply Ic must be otherwise furnished, and to this end a current source such as battery Ec, in series with a high resistor i0 and a choke'coil I l, is connected in shunt with the tuned circuit and the load 1. The input circuit is the same as in Fig. 4.

Because of the reactive properties of the tuned circuit, which reduces the effects of nonlinear distortion'at the operating frequency, this amplier may be biased substantially to collector voltagecutoff or beyond, thus greatly reducing the :steady-or direct current power consumed by thefload 1 in the-absence of a signal.

As =is well known, in a vacuum tube amplifier such as that or Fig. 3a. the combination of the resistorrl.,l connected between the grid and the cathode ofthetube and the condenser C1 connected-in-series lbetween the grid and the source operates automatically to 'supplement the negative grid bias once it has been overcome by the occurrence of an excessive positivesignal peak. This action ,takes place by reason of the fact that the current drawn by the grid operates to charge the condenser with such a polarity that the stored chargev actsto'A supplement the original steady negative bias. The situation dual to this is secured by the operation of the combination of the inductive element L. andthe resistor R1 in the Iinput circuit of the transistor ampliers of Figs.

B b, 4 and: 5. Upon-the occurrence of an excessive negative signal peak tending to drive the em1tter of the transistor below its (current) cutoff, thesource current isthen-alldravm through the inductance element L1 and the current thus For efficient action in this manner the impedance of the signal source which drives the transistor ampliiier should be high, thus being the dual counterpart of the source which drives the tube, whose impedance should be low.

The dual of a class B push-pull amplifier Fig. 7a shows a conventional class B vacuum tube push-pull amplifier with its deiining equations while Fig. 7b shows its transistor dual circuit together with the -corresponding equations. Duality appears in the choice of the character and magnitudes of the electrode biases. In order to obtain the high eiiiciency corresponding to class B operation, the emitters of the two transistors are biased toward high emitter current and the collectors toward high collector current, so that collector voltage is cut off during approximately one-half of each cycle. These bias conditions are illustrated in Fig. 6 where the family of characteristic curves for the one transistor are plotted back to back with those for the other transistor. In operation when =a signal to be amplilied is applied, for example by way of an input transformer l to the two emitters in push pull, the ampliiied version being correspondingly withdrawn by way of an output transformer I6, current and voltage swing approximately along a load line 2c (Fig. 6), while in the absence of a signal the collector voltages are both small compared to their values at the peaks of their swings. To avoid the distortion which would result from utilization of the highly non-linear parts of the characteristics which lie immediately adjacent to the current axis, the bias currents may be selected to locate the quiescent conditions of the two transistors approximately at the points P and P in Fig. 6.

The power supply arrangement shown in Fig. 7b satisfies the dual equations but is more elaborate than is necessary. As in the case of Figs. 3b and e the required operating currents can be derived from batteries and supplied by way of appropriate impedance elements in series with the input circuit and with the load. Fig. 8 Shows such an arrangement, more practical than Fig. 7b, in which a first battery 2|, choke coil 22 and resistor 23 are connected from the bases oi the two transistors to a center tap of the secondary winding of an input transformer 2i! to supply the emitter currents, while the collector currents are similarly supplied from a second battery 25 which is connected in series with a choke coil 2G and a resistor 2l from the bases of the transistors to a center tap on the primary winding of an output transformer 28.

Fig. 9 shows a modication of Fig. 8 in which, by appropriate proportioning of the resistors 23, 2l, and choke coils 22, 26, the emitter bias current le and the collector bias current Ic may both be supplied from a common source such as a battery 2d.

The push-pull transistor ampliiier ci' Figs. 8 `and 9 may be modiiied to supply power to load through a tuned circuit. Fig. 10 shows such a tuned ampliiier wherein, in accordance with the duality principle, the familiar antiresonant circuit of the tuned vacuum tube amplifier is replaced by a series resonant circuit in the output of the transistor amplier. This resonant circuit comprises an inductance element 3| and two series condensers 32, 32. Two condensers are recommended for reasons of balancing though, as far as tuning is concerned, a single condenser would serve as well. The operating collector currents may be supplied from a battery 2d by way of high resistors 23, 2, 2l and chokes 22, 25, 28. This supply arrangement combines the features of Fig. 5 with those of Fig. 9.

Because of the reactive properties of the tuned circuit load, which reduces the effects of nonlinear distortion at the operating frequency, this ampliner may be biased substantially to or beyond collector voltage cut 01T; i. e., it may be operated in a fashion which is dual to the operation oi a class B or a class C push-pull vacuum tube amplifier.

When more gain is required than can be obtained with a single stage transistor amplifier, two or more stages may be connected in cascade. The duality principle calls for feeding the collector current output oi an earlier stage to the emitter of the following stage, as compared with the voltage cascading which is familiar in the vacuum tube ampliiier art. When, in addition, it is desired that each stage shall be tuned, the duality principle calls for series-tuned resonant tuned circuits instead of the parallel-tuned or antiresonant circuits which are familiar in connection with tuned vacuum tube ampliers. Fig. ll shows a three-stage tuned push-pull amplifier which embodies both of these requirements. As in the case of Fig. i0, each stage of the ampliiler of Fig. 11 may be biased to operate on a class B basis and even on a class C basis. Here bias currents for the emitters and collectors may all be supplied. by way of choke coils and resistors from a common source lt in the manner shown in Fig. 9. The interstage coupling between the first stage and the second is secured by way oi a series resonant circuit comprising a coil il and a condenser i2 connected between the collector o1" the upper transistor of the rst stage and the emitter of the upper transistor of the second stage, while a similar circuit lll', d2 interconnects the corresponding electrodes of the lower transistors of the two stages. At the frequency to which these coupling circuits are tuned, their impedances are low compared with the collector contact impedances to which they are connected; and consequently, large signal frequency currents are delivered by the collectors of the transistors of the first stage to the emitters of the second stage, where they serve as second stage input currents. At other frequencies at which the impedances of the coupling circuits are high compared with the collector contact resistances to which they are connected, the collector currents so transferred from stage to stage are greatly reduced.

As an alternative, both input and output portions of any stage may be tuned, interstage coupling of any convenient variety being employed. As an example, Fig. 1l shows, in the case oi the second stage and the third, a series resonant tuned circuit for the output of the one and another series resonant tuned circuit for the input oi the other, the respective inductance coils of these tuned circuits being provided by the windings of an interstage transformer d5 whose turns ratio may be adjusted to give a desired impedance transformation.

lli

The series-tuned resonant circuit is but the simplest case ofamore general class of frequency selective networks whose impedances are low in the pass band and higher outside of it. It is contemplated that any network or filter of this class may be employed instead Of the simple seriestuned circuits shown. Av number of such networks are described in Transmission Networks and Wave Filters, by T. E. Shea (Van Nostrand, 1929), pp. 315 ff.

The same principles may be applied to an amplier of any number of stages, series resonant circuits or mole complex band pass filters being employed either to tune the input or the output of any stage, or to couple any stage to the stage which follows it. Aside from being more suitable for transistors than an antiresonant interstage coupling circuit, the series'resonant coupling circuits shown have the further advantage that they automatically prevent unwanted oscillations. Just as the vacuum tube is inclined to be .unstable when the impedances to which it is connected are too high, so the transistor is sometimes unstable when the impedances to which itis connected are too low. The impedances of the series resonant coupling circuits and of the more general class of lters to which they belong are higher at all frequencies other than the frequency to which they are tuned than they are at that frequency, and so tend to stabilize the transistors against undesired oscillations.

When the ampliiiers of Figs. 8, 9, 10 and l1 are driven by high impedance sources, the choke coilresistor combination by way of which the emitters of the transistors of these figures are connected to their bases operate in each case to provide automatic bias adjustment in the manner described above in connection with Fig. 3b.

The principles of the invention, which have been described as applied to amplifiers, are evidently equally applicable to circuits of other classes such as modulators, detectors, and the like. Thus, transistor oscillator circuits which embody the foregoing principles are the subject matter of an application of R. L. Wallace, Jr., Serial No, 184,459, filed September 12, 1950, and transistor multivibrator circuits embodying these principles are the subject of R. L. Wallace, 5r., Patent 2,620,448, which issued December 2, 1952,

on application Serial No. 184,458, filed September 12, 1950. Still other departures from the details of the illustrative embodiments shown are within the spirit of the invention.

What is claimed is:

1. A signal translating circuit which comprises a transistor having a semiconductive body, an emitter electrode, a collector electrode, and a base electrode engaging said body, an input circuit interconnecting said emitter electrode with said base electrode, an output circuit interconnecting said collector electrode with said base electrode, means for supplying to said emitter electrode a bias current which is substantially fixed in magnitude regardless of normal changes in the emitter electrode voltage vand for supplying to said collector electrode a bias current which is substantially xed in magnitude regardless of normal changes in collector electrode voltage, said means being proportioned to supply said .xed emitter bias current in magnitude such that, in the absence of a signal applied to said input circuit, the collector voltage of said transistor is substantially equal to zero, whereby application of an alternating input signal to said emitter electrode causes said collector voltage to swing away from its zero value and back again during signal ex'cursions'of 'one sign and'to' remain at said zero value throughout signal excursions of the opposite sign.

2. In combination with apparatus as defined in claim 1, a frequency-selective network connected in series with'said output circuit, said network having a relatively low impedance at a frequency to be translated and a relatively high impedance at higher and' lower' frequencies.

3. Apparatus as defined in claim 2 wherein thenetworkY comprises a series-tuned resonant circuit.V

4. In combination-with a signal source, a signal translating. apparatus as dened in claim l, of whichV the input'circuitis coupled with said source, wherein'V thev` iixed' emitter bias current supply means comprises a" resistor and an inductance coil connected in said input circuit, said resistorY being' proportioned in relation to thel emitter electrode'contact resistance to furnish said'xedemitterbias current in the absence of'y a` signal, saidv inductance coil being proportioned in'relation to the impedance of thesignal source to` automatically increase said emitter biascurrerita'bove said desired value in the presence Yof excessive signal' peak amplitudes.

5'. A multistage translating circuit of which each stage' comprisesa-translating'circuit as dened in claim l, asignal source coupled to the input circuit of the first stage, a load lcoupled to the output circuit of thelast stage, and a series resonant combination. of an inductive reactanceelement'and a capacitive reactance element directly connecting the collector oi" the transistor ofonest'age tothe emitter of the transistorof the'following stage.

6. A push-pull signaltranslating circuit which comprises a pairof transistors each having a semiconductivebody, an emitter electrode, a collector electrode; and' a" base electrode engaging said' body, said'basel electrodes being connected together, an: input' circuit4 interconnecting said emitter electrodesj an'4 output circuit interconnecting said-collector electrodes, means for supplying to said emitter electrodes bias currents which are substantially' fixed in magnitude regardless oftnor'mal changes in emitter electrode voltage and for supplying' to said collector electrodes\-loias` currents Which are substantially xed in magnitudev regardless of normal changes in collector electrode voltage; said means being proportioned' to-supply'said xed emitter bias currents in magnitudes such'that, inthe absence of a signal applied to said input circuit, the collector voltages` of said transistors'are individually substantiallyequal to Zero,whereby-application of an alternating* inputv signal toV said emitter cleotrodescauses' said-collectorvoltages to swing away from their zero values andi backagain in alternation, ea'chremainin'gat its1zero value throughout signal excursionsl of one sign.

'7. Incombination-'with apparatus as defined in claim 6, a frequency'-selefctiveV network connected in Series Withsaid collector' electrodes, said network having a'relatively` low` impedance at a frequency to betranslated' and a relatively high irnpedance at'higher and lower frequencies.

8. Apparatus: as dened in claim 7, wherein the network comprises a series-tuned resonant circuit.

9. In` combination with a signal source, signal translating. apparatus as defined in claim 6, of which the input circuit is coupled with said source, wherein the ,fixed emitter bias current supply means comprises a resistor and an inductance coil connected in said input circuit, said resistor being proportioned in relation to the emitter electrode contact resistances to furnish said xed emitter bias currents in the absence of a signal, said iriductance coil being proportioned in relation to the impedance of the signal source to aun tornati-cally increase said emitter bias currents above said desired value in the presence of excessive signal peak amplitudes.

19. A multistage push-pull translating circuit of which each stage comprises a translating circuit as defined in claim 6, a signal source coupled to the input circuit of the irst stage, a load coupled to the output circuit of the last stage, and a series resonant combination of an inductive reactance element and a capacitive reactance element directly connecting the collector of one transistor of one stage to the emitter of one transistor of the following stage.

ROBERT L. WALLACE, JR.

i4 References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,502,479 Pearson et al. Apr. 4, 2,517,960 Barney et al. Aug. 8, 1950 2,524,035 Bardeen et al. Oct. 3, 1950 2,541,322 Barney Feb. 13, 1951 2,569,347 Shockley Sept. 25, 1951 OTHER REFERENCES 

